In recent years, an increasing number of opthalmic lenses for the correction of ocular disorders have been fabricated by casting and curing a transparent plastic material, such as allyl diglycol carbonate monomer. These cast lenses are generally lighter than comparable glass lenses, and the casting process involves far less labor than the equivalent grinding and polishing process required for glass lenses.
Generally speaking, the lens casting process is carried out by means of a concave and convex die supported in spaced apart relationship by an optical casting gasket. The die surfaces determine the optical characteristics of the cast lens, and are accordingly prepared with great care and accuracy. As a result, the casting dies themselves represent a substantial expense.
There are a number of optical characteristics which must be carefully controlled in a lens to satisfy corrective ophthalmic requirements, generally comprising any one or more of the following: sphere power, cylinder power, cylinder axis orientation, reading segment type and location, prism power and direction, optical center location, diameter, and thickness. These characteristics each may vary over a wide range. To manufacture finished ophtalmic lenses which account for all the variations in all of the parameters is impractical. Indeed, even the manufacture of finished lenses which fulfill the most frequently prescribed corrective needs require an enormously large inventory.
Consequently, it is common practice to manufacture ophthalmic lenses as blanks, wherein one surface is cast with a general corrective curvature and the other surface is subsequently ground and polished to meet the particular corrective requirements. The grinding and polishing procedures required to finish the blanks are time consuming and expensive, whereas the cost of casting the blanks is rather low. By comparison, were it possible to cast plastic lenses in finished form, the manufacturing cost per lens would be quite low. However, finish casting of plastic lenses is merely an ideal in the state of the art, due to the enormous number of casting gaskets and lens casting dies that would be required to produce the range of frequently prescribed corrective lenses.
Previous proposals have been made for lens casting assemblies in which attempts have been made to avoid or minimize the problems referred to above. Such proposals include, e.g., U.S. Pat. No. 3,946,982 in the name of Calkins et al., and U.S. Pat. No. 3,056,166 in the name of J. W. Weinberg; both these specifications still involve the use of many different types of gasket. In the case of U.S. Pat. No. 3,056,166 a fresh gasket is cut from a pre-form each time, and U.S. Pat. No. 3,946,982 while reducing the number of individual gaskets required, still requires a substantial number. In both cases, problems would be expected in preventing leakage of casting material out of the assembly due to the nature of the gaskets and the way the assembly is formed.